The subject matter discussed in the background section herein should not be assumed to be prior art merely as a result of its mention in the background section. Similarly, a problem mentioned in the background section or associated with the subject matter of the background section should not be assumed to have been previously recognized in the prior art. The background section may rely on hindsight understanding and may describe subject matter in a manner not previously recognized in the prior art, and it should not be assumed that such descriptions represent the understanding or motivations of those skilled in the art before the filing of this application. The subject matter in the background section merely represents different approaches, which in and of themselves may also be inventions.
A fluid device may include any device that moves fluids or uses moving fluids. Two examples of a fluid device include a pump and a motor. A pump is a device that moves fluids (e.g., liquids, gases, slurries) using mechanical action. A motor is a device that converts energy received from fluids into mechanical action.
Pumps and motors may both use pistons to control fluid movement. A piston is a reciprocating component that allows fluid to expand in a chamber during an up stroke and compresses and/or ejects the fluid during a down stroke. In a pump, force may be transferred from the crankshaft to the piston for purposes of compressing or ejecting the fluid. In a motor, force may be transferred from the fluid to the piston for purposes of rotating the crankshaft. In some fluid devices, a piston may also act as a valve by covering and uncovering ports in a chamber wall.
In one example, a piston is a cylindrical component that utilizes a close tolerance cylindrical fit between the piston and a cylinder bore chamber to minimize efficiency loses from internal leakage. The term “cylinder” and its variants may refer to a general cylindrical shape represented by points at a fixed distance from a given line segment, although in practice cylinders may not be perfectly cylindrical (e.g., due to manufacturing constraints) and may include non-cylindrical cavities, passageways, and other areas.
Some fluid devices may be classified as fixed displacement or variable displacement. In a fixed-displacement fluid device, displacement distance of each piston stroke remains constant, and fluid flow through the fluid device per rotation cannot be adjusted. In a variable displacement fluid device, fluid flow through the fluid device per rotation may be adjusted by varying the displacement distance of each piston stroke.
In some fluid devices, pistons are arranged axially such that their piston stroke centerlines are configured in a circle parallel to the rotational axis of the cylinder block centerline. FIG. 1 shows a cross-section of an example axial fluid device 100. Axial fluid device 100 features a shaft 110, a cylinder block 120, a swashplate 130, pistons 140, and a pressure compensator 150. Pistons 110 may reciprocate within cylinders of cylinder block 120. Swashplate 130 allows energy to be converted between the rotary motion of shaft 110 and the linear motion of pistons 140. Swashplate 130 drives each piston 110 through one sinusoidal stroke motion for each revolution of shaft 110. A sinusoidal stroke includes one “up stroke” motion and one “down stroke” motion.
In a fixed-displacement fluid device, the angle of swashplate 130 is fixed. In a variable-displacement fluid device, pressure compensator 150 may vary the angle of swashplate 130 to change displacement and direction. To minimize the load required to change the angle of swashplate 130 in variable-displacement fluid devices, the diameters of pistons 110 may be kept small, and the pivot axis of swashplate 130 may be offset from the rotation axis of cylinder block 120 to allow forces from pistons 110 to counterbalance the load.
In other fluid devices, pistons are arranged radially such that their piston stroke centerlines are configured radially outward from the rotation axis of the cylinder block. FIGS. 2A and 2B show cross-sections of an example radial fluid device 200. Radial fluid device 200 features a shaft 210, a cylinder block 220, a cam 230, pistons 240, and pressure compensator 250. In this example, pressure compensator 250 may vary the displacement and direction of pistons 240 by varying the offset of the centerline of cam 230 relative to the centerline of cylinder block 220. The load required to move cam 230 is relatively high because the configuration has a high piston diameter to stroke ratio compared to axial designs and there are no forces available to counterbalance the piston loads acting on the cam. Thus, pressure compensator 250 must be large enough to provide the force necessary to move cam 230.
In the example of radial fluid device 200, cam 230 is circular. In this example, circular cam 230 may be referred to as a single-lobed cam because it causes pistons 240 to complete only one sinusoidal stroke per rotation of cylinder block 220. Cams having more than one lobe, such as an elliptical (two-lobed) cam, do not typically lend themselves to being offset to vary displacement because of their unique shape.
In the example of FIG. 2, radial fluid device 200 varies fluid flow by varying piston stroke displacement. As explained above, such an arrangement requires a significant amount of force to move cam 230. In an alternative approach, fluid flow may be varied by varying valve timing. For example, U.S. Patent Publication No. 2011/0220230 describes a radial pump with fixed piston displacement and independent electronic intake valve control. Varying valve timing may require more energy to open and close each valve, however. In particular, varying valve timing may require closing the inlet valve and opening the outlet valve at points in the piston stroke where hydraulic flow is at a maximum.